This application claims the benefit of Japanese Patent Application No. 2008-104591, filed Apr. 14, 2008, which is hereby incorporated by reference herein in its entirety.
1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer having a sheet feeding apparatus which feeds a sheet.
2. Description of the Related Art
A conventional image forming apparatus is provided with a sheet feeding apparatus which feeds a sheet to an image forming portion. Especially in a commercial-use mass-printing image forming apparatus, since a sheet feeding operation to the image forming portion largely influences the productivity (the number of sheets to be printed per unit time), it is extremely important to stably and reliably feed sheets by the sheet feeding apparatus.
In separating mechanisms in the sheet feeding apparatuses which enhance the productivity and reliably feed sheets, retard separating systems are widely used. The retard separating system feeds stacked sheets by a pickup roller, separates the sheets individually and conveys a sheet between a feed roller which rotates in the same direction as a sheet feeding direction and a retard roller which can rotate in a direction opposite from the sheet feeding direction.
In the retard separating system, the retard roller receives a torque in a direction opposite from the sheet feeding direction through a torque limiter, and when one sheet is fed by the pickup roller, a driving force is cut by the torque limiter and the retard roller rotates so as to follow the sheet.
When two or more sheets are fed, since a friction coefficient between the sheets is smaller than a friction coefficient between the feed roller and the sheets, the retard roller rotates in the opposite direction from the sheet feeding direction to return the second and subsequent sheets.
The friction coefficients of a sheet and the roller are changed depending upon the water content of the sheet to be fed and presence or absence of extraneous matter on a surface of the sheet. Because of this, overlapping-feeding in which sheets are not separated from each other and two or more sheets are superposed and fed, or misfeeding (feeding failure), in which a sheet is not fed by the separating mechanism, may be generated in some cases.
When a new sheet bundle is added to a sheet bundle which is stored in a cassette, this overlapping-feeding is especially prone to being generated due to extraneous matter or a difference in water content of sheets around the boundary between the previously stored sheet bundle and the newly added sheet bundle.
Further, sheets having high smoothness, such as art paper or coated paper used for color printing, are crushed near a boundary between sheet bundles and it becomes difficult for air to enter between the sheets. As a result, adsorption force between the sheets is increased, and the overlapping-feeding is prone to being generated near the boundary between the sheet bundles. If the overlapping-feeding or misfeeding of sheets is generated, it is necessary to stop the image forming apparatus to take out the overlapping-fed sheet or misfed sheet, and the productivity is deteriorated correspondingly.
It is proposed to automatically change a nip pressure of a separating mechanism comprising a feed roller and a retard roller and an abutment pressure of a pickup roller on sheets when the overlapping-feeding or misfeeding of sheets is generated. This technique is disclosed in Japanese Patent Application Laid-open No. 05-32356.
In this proposal, when the overlapping-feeding of a sheet is detected by an overlapping-feeding detection sensor, a nip pressure of the separating mechanism is increased to enhance the separating degree. Further, when a sheet is not sent to a predetermined position within a given time, it is determined that misfeeding of sheets is generated, and, therefore, a sheet abutment pressure of the pickup roller is increased and the conveying force of a sheet is enhanced. With this, the overlapping-feeding or misfeeding of a sheet is prevented.
In the conventional image forming apparatus, however, when sheets are overlapping-fed, the conveying operation of sheets is stopped, and the overlapping-fed sheet must be removed from the image forming apparatus. Therefore, even if the nip pressure of the separating mechanism is adjusted to solve the overlapping-feeding of the next sheet, although it is possible to prevent the overlapping-feeding from being generated, the already generated overlapping-fed sheet cannot be solved.
That is, in the conventional proposal, when a roller is worn with time or when special sheets having friction coefficient different from that of plain paper are continuously fed and the overlapping-feeding is frequently generated, the nip pressure is adjusted to stabilize the feeding operation of sheets. Therefore, this proposal is not suitable for overlapping-feeding caused due to change in temporary surface states of sheets.
That is, a problem such as the overlapping-feeding of sheets prone to be generated in a boundary between sheet bundles cannot be solved, and it is, therefore, difficult to maintain high productivity.